Hermetic package for an electronic device and method of manufacturing same

ABSTRACT

A hermetic package for an electronic device is manufactured by providing a green glass ceramic body with a green via to produce a workpiece. The workpiece is sintered at a temperature at or above 500°  C., while compressing the workpiece at a pressure at or above 100 pounds per square inch, so as to obtain a hermetic package. The green via comprises a mixture of copper and a glass ceramic material with a sufficient volume of glass to produce a hermetic package, yet with sufficient copper to have a suitable electrical conductivity. 
     The hermetic package thus produced comprises a sintered glass ceramic body having an electrically conductive sintered via which is hermetically bonded to the glass ceramic body and which comprises a mixture of an electrically conductive material and a glass ceramic material. The electrically conductive material forms at most 50 volume percent of the via. 
     The workpeice may be sintered in a sintering fixture having a frame and a compensating insert. The compensating insert and frame bound a sintering chamber for accommodating the workpiece. By providing a frame having a thermal expansion coefficient greater than that of the workpiece, and by providing a compensating insert having a thermal expansion coefficient greater than that of the frame, a close fit can be assured between the workpiece and the sintering fixture over a large range of temperatures.

This is a division of Ser. No. 07/418,435 filed Oct. 6, 1989, and isrelated to Ser. No. 07/757,747, entitled "Hermetic package for aelectronic device," filed Sep. 11, 1991.

BACKGROUND OF THE INVENTION

The invention relates to hermetic packages or hermetic substrates forelectronic devices. The invention also relates to methods and apparatusfor manufacturing such hermetic packages or substrates.

Many electronic devices, such as semiconductor integrated circuits, mustbe packaged in clean inert atmospheres for obtaining reliable operationover long lifetimes. In addition to being hermetic, such packages mustprovide electrical connections between the circuits inside the packageand external devices outside of the package. Often the package alsoprovides electrical connections between different points on the circuitor circuits inside the package. Such a package may comprise, forexample, a hermetic substrate sealed to a hermetic lid by way of aflange so as to form an enclosed chamber containing one or moreelectronic devices.

One type of known electronic device package (see, for example, U.S. Pat.No. 4,234,367) consists of a multilayer glass ceramic substrate. On oneside, the substrate has termination pads for attaching electronicdevices. On the other side, the substrate has termination pads formaking external connections.

Each layer of glass ceramic may be provided with one or moreelectrically conductive thick film lines on the surfaces of the layer,and one or more electrically conductive vias passing through the layer.The vias connect electrically conductive thick film lines and/ortermination pads on opposite surfaces of the layer. Vias which extend totermination pads on the substrate surface must be accurately positionedat locations corresponding to the locations of terminals on theelectronic devices to be packaged.

Such multilayer glass ceramic substrates are manufactured by producing aslurry of glass particles in a binder. The slurry is cast and dried intogreen sheets. Via holes are punched through the green sheets in desiredconfigurations, and a copper paste is extruded into the via holes. Acopper paste is also screen printed onto the green sheets in a desiredconductor pattern to form line interconnections and voltage planes. Aplurality of sheets are laminated by pressing above the glass transitiontemperature of the green sheets (typically 70°-100° C.). Finally, thelaminated sheets are sintered.

After sintering, the substrates may exhibit certain structuralirregularities that may adversely affect the hermeticity of thesubstrate. Consequently, it has been known to "back fill" the gaps andcracks in the substrate with a polymer or other sealing material aftercompletion of sintering, in order to obtain a hermetic substrate. Suchan additional processing step is, however, costly and time consuming.

After the substrate is "back filled", the opposite surfaces of thesubstrate must be ground and polished flat and parallel to each other toprovide suitable mounting surfaces for electronic components andexternal connections. It is important that the conductive vias benondistorted so that termination pads are properly aligned withelectronic components and external connections to be mounted on thesubstrate.

SUMMARY OF THE INVENTION

It is an object of the invention to produce a sintered multilayer glassceramic substrate having electrically conductive vias containing ametal, which substrate is hermetic without post-sintering processing.

It is another object of the invention to produce a sintered multilayerglass ceramic substrate having electrically conductive vias, whichsubstrate has substantially no structural irregularities.

It is a further object of the invention to produce a sintered multilayerglass ceramic substrate having nondistorted electrically conductivevias.

According to the invention, a hermetic package for an electronic devicecomprises a sintered glass ceramic body which is substantiallyelectrically insulating. An electrically conductive sintered via in theglass ceramic body extends from the first surface of the glass ceramicbody to the second surface of the glass ceramic body. The via ishermetically bonded to the glass ceramic body. The via comprises amixture of an electrically conductive material and a glass ceramicmaterial. The electrically conductive material forms at most (50) volumepercent of the via.

In a method according to the present invention of manufacturing ahermetic package for an electronic device, a green glass ceramic body isprovided with a green via extending through the body from the firstsurface to the second surface of the body. The green via comprises amixture of an electrically conductive material and a glass ceramicmaterial. The workpiece formed by the via-containing glass ceramic bodyis compressed at a pressure at or above 100 pounds per square inch whilesintering the workpiece at a temperature at or above 500° C. so as toobtain a hermetic package.

The present invention is advantageous because by pressure sintering agreen glass ceramic body containing green vias comprising a mixture ofelectrically conductive material and a glass ceramic material, the viaswill hermetically bond to the glass ceramic body. Consequently, apost-sintering sealing step will not be needed.

Preferably, in the method according to the present invention theworkpiece is sintered at a pressure which is sufficiently high that thepressure sintering densification rate is much greater than the freesintering (unpressurized sintering) densification rate.

It is also preferred, according to the present invention, that the greenvia (i.e. prior to sintering) consists essentially of 20 to 50 volumepercent copper, and 80 to 50 volume percent glass ceramic material.

In order to assure that the vias passing through the glass ceramic bodyremain undistorted with respect to the surfaces of the glass ceramicbody, edge distortion must be avoided during pressure sintering.According to an aspect of the present invention, edge distortion isavoided by varying the pressure on the workpiece during sintering as afunction of the thickness and the edge profile or width of theworkpiece. Preferably, the workpiece is compressed at a pressure, P,substantially given by the equation ##EQU1## where γ is the surfacetension of the workpiece, D is the ratio of the density of the workpiecedivided by the theoretical maximum density of the workpiece, R is theradius of the workpiece, h is the height of the workpiece, and n is thepore density in the workpiece.

In another aspect of the invention, the workpiece is compressed untilthe workpiece has a selected thickness.

Edge distortion of the workpiece can be avoided, according to anotherembodiment of the invention, by providing a sintering fixture to supportthe edges of the workpiece during pressure sintering. In this embodimentof the invention, a sintering fixture comprises a frame and acompensating insert arranged inside the frame. The frame has a thermalexpansion coefficient α_(A), and the compensating insert has a thermalexpansion coefficient α_(B). The compensating insert and the frame bounda sintering chamber for accommodating the workpiece. In order to avoidcrushing or distorting the workpiece on cooling after completion of thepressure sintering, since the thermal expansion coefficient α_(C) of theworkpiece is less than the thermal expansion coefficient α_(A) of theframe, the thermal expansion coefficient α_(B) of the compensatinginsert is made much greater than the thermal expansion coefficient α_(A)of the frame.

Preferably, the frame has a length l_(A) in a first direction, thecompensating insert has a length 21l_(B) in the first direction, and theworkpiece has a length l_(C) in the first direction such that ##EQU2##where 2d is a desired gap between the fixture and the workpiece at roomtemperature, and where δT is the difference between room temperature andthe pressure sintering temperature.

The sintering fixture according to the present invention is advantageousbecause by suitable choice of the thermal expansion coefficients of theworkpiece, the frame, and the compensating insert, the edge of theworkpiece will be supported by the sintering fixture in a temperaturerange from room temperature through the maximum pressure sinteringtemperature.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a cross-sectional view of a hermetic package according to thepresent invention.

FIG. 2 is a cross-sectional view of another embodiment of a hermeticpackage according to the present invention.

FIG. 3 is a schematic view of a pressure sintering apparatus for use ina method according to the present invention of manufacturing a hermeticpackage.

FIG. 4 is a schematic illustration of the manufacture of a hermeticpackage by pressure sintering.

FIG. 5 is a cross-sectional view of an apparatus for use in anembodiment of the method according to the present invention formanufacturing a hermetic package.

FIG. 6 is a top plan view of a sintering fixture according to thepresent invention for pressure sintering a workpiece to form a hermeticpackage.

FIG. 7 is a plot of resistivity versus sintering pressure for severalvia paste compositions according to the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 1 shows a simple hermetic package for an electronic device. Thehermetic package comprises a sintered glass ceramic body 10. Thesintered glass ceramic body 10 has first and second opposite surfaces 12and 14. Animaginary axis 16 extends from the first surface 12 to thesecond surface 14. The glass ceramic body has a thickness or height, h,in the direction of the axis 16. The glass ceramic body 10 also has anedge 18 which connects the surfaces 12 and 14. The glass ceramic body issubstantially electrically insulating.

The hermetic package also comprises at least one electrically conductivesintered via 20 in the glass ceramic body 10. The via 20 is hermeticallybonded to the glass ceramic body 10. The via comprises a mixture of anelectrically conductive material and a glass ceramic material. Theelectrically conductive material forms at most 50 volume percent of thevia.

Examples of glass ceramic materials which may be used to form the glassceramic body 10 are given in U.S. Pat. No. 4,234,367 and U.S. Pat. No.4,301,324, the disclosures of which are incorporated by referenceherein. Cordierite glasses, mullite glasses, or other glasses which canbe heated to crystallization can also be used. Mixtures ofnoncrystallizing glasses and ceramics may also be used. For purposes ofillustration, and not of limitation, several glass ceramic compositionsthat may be used according to the present invention are listed in TableI.

                                      TABLE I                                     __________________________________________________________________________    GLASS CERAMIC COMPOSITIONS (WEIGHT PERCENT)                                   1      2  3  4  5  6  7  8  9  10 11 12  13  14                                                                            15 16 17 18 19 20                __________________________________________________________________________    SiO.sub.2                                                                         55.0                                                                             54.5                                                                             54.5                                                                             52.3                                                                             57.8                                                                             60.0                                                                             50.0                                                                             53.9                                                                             54.0                                                                             55.0                                                                             60.0                                                                             54.5                                                                             57.0 47.00                                                                            53.9                                                                             54.0                                                                             54.5                                                                             54.0                                                                             54.0              Al.sub.2 O.sub.3                                                                  21.1                                                                             21.0                                                                             21.0                                                                             19.7                                                                             22.2                                                                             18.2                                                                             22.9                                                                             20.7                                                                             21.0                                                                             21.0                                                                             17.0                                                                             20.0                                                                             21.0 33.50                                                                            20.8                                                                             22.0                                                                             21.3                                                                             22.0                                                                          21.0                 MgO 22.3                                                                             19.8                                                                             19.8                                                                             24.0                                                                             16.0                                                                             17.8                                                                             22.2                                                                             19.6                                                                             19.0                                                                             18.0                                                                             18.0                                                                             20.0                                                                             20.0 13.50                                                                            19.6                                                                             20.0                                                                             19.9                                                                             20.0                                                                          22.0                 B.sub.2 O.sub.3                                                                   1.2                                                                              1.0                                                                              1.0                                                                              1.0                                                                              1.0                                                                              1.0                                                                              1.1                                                                              1.0                                                                              1.0                                                                              1.0                                                                              1.0                                                                              1.0                                                                               1.0 3.00                                                                             1.0                                                                              1.0                                                                              1.0                                                                              1.0 1.0              P.sub.2 O.sub.5                                                                   0.3                                                                              2.7                                                                              2.7                                                                              3.0                                                                              3.0                                                                              3.0                                                                              1.8                                                                              2.8                                                                              3.0                                                                              3.0                                                                              3.0                                                                              1.5                                                                               1.0 3.0                                                                              2.7                                                                              2.0                                                                              2.8                                                                              2.0                  CeO.sub.2                                                                            1.0                                                                    MnO       1.0                                                                 ZrO.sub.2                2.0         2.0                                      CaO                         2.0      1.0                                      NiO                            2.0                                            Li.sub.2 O                        1.0                                         Fe.sub.2 O.sub.3                                2.0                           Na.sub.2 O                                         1.0                        TiO.sub.2                                             1.0                     ZnO                                                      1.0                  CuO                                                          2.0              __________________________________________________________________________

Examples of materials which may be used to form the via are any of theglass ceramic materials described above, mixed with 20 to 50 volumepercent copper, gold, palladium, silver, nickel, or alloys thereof.

In order to provide a good electrical connection through the hermeticpackage, the sintered via 20 preferably has an electrical resistivitylessthan 1000 microohm-centimeters.

FIG. 2 shows an example of a more complex hermetic package comprising atrilayer sintered glass ceramic body 10. A number of electricallyconductive sintered vias 20 are provided through the various layers ofglass ceramic body 10. In this example, the hermetic package furtherincludes electrically conductive thick film lines 22 for electricallyconnecting one or more vias 20 to each other. The exposed surfaces ofvias20 form terminations for connection to electronic devices to bepackaged, or for external connections.

The thick film lines 22 may comprise, for example, a copper paste orink. Alternatively, the lines 22 may comprise other metals such assilver or gold or alloys thereof.

A hermetic package, such as those shown in FIGS. 1 and 2, ismanufactured according to the present invention by first providing agreen (unfired) glass ceramic body which is substantially electricallyinsulating, and providing one or more green (unfired) vias in the greenglass ceramic body. The green glass ceramic body and the green vias froma workpiece 24 as shown in FIG. 3. According to the present invention,the workpiece 24 is sintered at a temperature at or above 500° C. At thesame time the workpiece 24 is compressed at a pressure at or above 100pounds per square inch while sintering the workpiece, so as to obtain ahermetic package.

Sintering refers to the process of transforming the initially highlyporousworkpiece into a highly dense module. When the workpiece issintered without being compressed, the workpiece has a free sinteringdensificationrate. When the workpiece is sintered while beingcompressed, it has a pressure sintering densification rate. According toa first embodiment of the invention, the pressure sinteringdensification rate is much greater than the free sintering densificationrate.

In order to obtain a hermetic bond between the via and the glass ceramicbody, it is preferable that the green via consists essentially of atleast50 volume percent glass ceramic material. However, in order tomaintain a sufficiently high electrical conductivity, the green viashould contain not less than 20 volume percent copper.

A first embodiment of the method according to the present invention formanufacturing a hermetic package is shown in FIG. 3. The workpiece 24 iscompressed between upper die 26 and lower die 28.

While the workpiece 24 is compressed between dies 26 and 28, it isimportant that the edges 30 of the workpiece 24 are not deformed.Therefore, according to the invention, means 32 are provided formeasuringthe thickness of the workpiece, means 34 are provided formeasuring the width or edge bulge 30 of the workpiece, and means 36, 37and 38 are provided for varying the pressure on the workpiece as afunction of the measured thickness and as a function of the measurededge bulge (or width)of the workpiece.

The means 32 and 34 for measuring the thickness and edge of theworkpiece may be linear variable displacement transducers (LVDT's). Thepressure controller 36 may be, for example, a general purpose digitalcomputer for controlling a programmable pressure regulator and pump 37,which in turn applies pressure to workpiece 24 by way of pressure cell38. The pressure may be supplied by a hydraulic press 38. The workpiece24 and the dies 26 and 28 are arranged inside a furnace 40,schematically illustrated.

In the method, a programmable temperature controller 48 controls thesintering temperature according to a preselected temperature schedule.Temperature controller 48 measures the furnace temperature by way ofthermocouple 50, and regulates the furnace temperature by way of powersource 52 and heater element 54.

Deformation of the edges 30 of the workpiece 24 may be avoided duringpressure sintering by varying the pressure as a function of thethickness,edge bulge, and relative density of the workpiece. Thisfunctional relationship can be derived in the following manner.

Consider the sintering of a circular, porous glass ceramic module (FIG.4) of initial density, ρ_(O), initial thickness, h_(O), and constantradius, R, under a pressure schedule P[t] and a temperature scheduleT[t] where t is the time elapsed after the start of the experiment.

For the sintering of ceramics, the Reynolds number of the flow is sosmall (due to the very high glass viscosities, e.g., >10⁹ poise for ourglass ceramics) that the inertia terms in the equations of motion arenegligible. Also, because the aspect ratio, R/h (h being the thicknessof the module at any time t), is typically much greater than unity, thetwo flow velocities, V_(r) and V_(z), satisfy the simplifiedNavier-Stokesequation ##EQU3##and the continuity equation ##EQU4##basedon the assumption that the glass ceramic is a Newtonian substance.InEquations 1 and 2, r and z signify the lateral and axial directionsshown in FIG. 4, p is the dynamic pressure defined as the pressure abovethe ambient, η is the viscosity, and ρ is the module density which isassumed to vary only with time. The viscosity can depend on temperature,and time if crystallization is involved (see Equation 6 below).

The initial and boundary conditions associated with Equations 1 and 2are typical of squeeze-flow situations:

    ρ=ρ.sub.0 at t=0                                   (3a)

    h=h.sub.0 at t=0                                           (3b)

    v.sub.z =0 at z=0                                          (3c) ##EQU5##

    v.sub.r =0 at z=0, h                                       (3e)

    p=0 at r=R                                                 (3f) ##EQU6##

The mathematical system defined by Equations 1-3 contains two unknowns,namely, the viscosity, η, and the density, ρ. To determine the densityvariation with time during sintering, an expression covering a widerange of pressures has been proposed by Murray et al ("Practical andTheoretical Aspects of the Hot Pressing of Refractory Oxides,";Transactions British Ceramic Society; Volume 53, 1954, pages 474-510)for molding applications: ##EQU7##where D=ρ/ρf, ρf is the fully sintereddensity, γ is the surface tension, and n is the number of pores per cm³of the module which is intimately connected to the particle size of themodule. For our glass ceramics, γ˜360 dynes/cm (Giess et al. J. Amer.Ceram. Soc., 1984, Vol. 67, pages 549 et seq.), and n is assumed to beidentical to that reported by Murray et al., (i.e., n=1.58×10⁸ /cm³)dueto lack of data.

We shall use Equation 4 to calculate the density under either isothermalordynamic heating conditions with the understanding that both thepressure and viscosity can vary with time. Because the local pressureeffective forsintering is the hydrodynamic pressure which varies in thelateral direction, r, due to the absence of a die, we shall replace P inEquation 4 with the effective pressure ##EQU8##which will be evaluatedlater.

In a manner analogous to that occurring during curing of a polymer, theviscosity, η of a glass ceramic can be represented by the dual-Arrheniusexpression (Roller, M. B. "Characterization of theTime-Temperature-Viscosity Behavior of Curing B-Staged Epoxy Resin",Polymer Eng. Sci., Jun. 1975, Vol. 15, No. 6, pages 406-414):##EQU9##where R' is the universal gas constant and

η∞, ΔE.sub.η, K∞ AND ΔE_(k) are constant parameters. This expressionaccounts for both an exponential viscosity decay with temperature due tosoftening of the glass (first two terms on the right-hand side), and alinear viscosity increase due to crystallization according to afirst-order kinetics (the last term on the right-hand side).

It must be noted that the last two terms on the right-hand side requireknowledge of the thermal history of the module for their evaluation.Thus,interestingly enough, even two fluid elements having the sametemperature at some time could have very different viscosities if theirtemperatures at previous times were not identical.

Whenever a temperature non-uniformity exists in the ceramic module thetrajectory of each fluid element must be evaluated, in general, todetermine the spatial distribution of the viscosity of the glassceramic. This can be done, in principle, by solving the energyconservation equation together with the equations of motion for a givenset of initial and boundary conditions. In this study, however, it wasassumed that the viscosity at a given time does not depend on positionor, equivalently, the temperature which governs the viscosity isuniform.

The parameters in Equation 6 can be obtained by, for example, performingsome isothermal parallel-plate rheometry experiments at temperaturescovering the temperature range of interest using reasonably non-poroussamples. (Tong et al. "Prediction of Thermoset Viscosity Using aThermomechanical Analyzer", Journal of Applied Polymer Science, 1986,Vol.31, pages 2509-2522.) Once the parameters are known, Equation 6 canbe usedto predict the viscosity history under a wide variety oftemperature schedules. When the effect of crystallization on theviscosity has not been established fully, it is necessary to resort tothe viscosity-temperature relationship up to some maximum temperaturewhere the crystallization effect is likely to become important.

Solving for the mathematical system (Equations 1-3), we obtain##EQU10##indicating that the sample thickness, h, is controlled by thecompetition between a lamination flow term (first term on the right-handside) which is proportional to the ratio of the applied pressure, P, tothe sample's cross sectional area, A(=.sup.πR2), and a sintering flowterm (second term on the right-hand side) which depends on the poreclosing pressure (or the applied pressure) but not on the sample area.Therefore, when the lamination flow term is dominating, one expects thethickness to depend onthe pressure to area ratio. On the other hand,when the sintering flow termdominates, the thickness is insensitive tothe sample area. Accompanying Equation 7 are P=4P/3 and the velocityprofiles ##EQU11##Note here that the expression of V_(z) contains asintering or density contribution (first term on the right-hand side)and a lamination contribution. The sintering contribution disappears inthe expression of V_(r), i.e., the lamination flow. This is due to themuch smaller sintering flow in the lateral direction compared to that inthe thickness direction--a direct manifestation of immobile modulesurfaces. According to V_(r) alone, the edge always bulges outward. Thisis inconsistent with the shrink-in edge observed at low pressures (e.g.,<5 psi.) for our glass ceramics. The discrepancies here were due to theincreasing importance of the lateral sintering flow at low pressures. Noexpression is yet available to describe this retarded sintering flow inthe lateral direction. As an approximation, we assume that at the centerbetween the two surfaces of the module (i.e., at z=h/2 and r=R), thematerial sinters or shrinks in accordance with the free sintering law ata speed, λ where ##EQU12##If one allows the edge to shrink conformally(i.e., at all z's) according to Equation 10 instead of just at thecenter (i.e., if λ=R with R now varying with time) and if Equation 10also governs the thickness changes (i.e., λ=h), then Equation 10 isequivalent to Equation 4 with P=0. With λ, the edge bulge, θ, is givenby ##EQU13##

Depending on the sign of θ, the edge either bulges outward (θ>0) orshrinks inward (θ<0). According to Equation 11, the condition for a flatedge (θ=0) rests upon setting V_(r) =λat all times. This yields thepressure schedule ##EQU14##where R_(o) is the initial radius of theworkpiece. From Equation 12, it can be seen that the pressure schedulerequired for a flat edge depends onthe thickness, h, the edge bulge, θ,and the relative density, D.

Thus, in an embodiment of the invention, the LVDT 32 measures thethicknessof the workpiece 24, and the LVDT 34 measures the width or edgebulge of the workpiece 24. The relative density D is calculated bycontroller 36 based on the known mass of the workpiece and based on themeasurements of h and θ.

Pressure controller 36 varies the pressure on the workpiece 24 as afunction of the measured thickness, the measured edge bulge, and therelative density of the workpiece according to the pressure schedule ofEquation 12.

When the measured edge bulge θ exceeds a selected limit, controller 36releases the pressure on workpiece 24. This allows free sintering toreduce the edge bulge. When the edge bulge is back within the selectedlimit, the pressure is reapplied according to Equation 12. When thedesired sample thickness is reached, controller 36 releases the pressurefor the remainder of the process.

Edge deformation can be minimized in another aspect of the invention byproviding stops 42 between the dies 26 and 28. (FIG. 3.) By compressingthe workpiece 24 until the upper die 26 contacts the stops 42 and theworkpiece has a selected thickness, edge deformation can be minimized.

It is important to avoid edge deformation in hermetic packagescontaining vias, because edge deformation is also associated with viadeformation. Consequently, when edge deformation is avoided, then viadeformation is also avoided.

Deformed vias will not properly align with either termination padsdeposited on the tops of the vias, or electrical contacts on electroniccomponents to be mounted on the hermetic package.

As shown in FIG. 3, when the edges 30 of the workpiece 24 are supportedonly by the workpiece itself in a direction transverse to the axis 16 ofthe workpiece, edge deformation is avoided by suitably controlling thepressure on the workpiece. In another embodiment of the invention,however, a sintering fixture is provided for supporting the edges 30 ofthe workpiece.

Referring to FIGS. 5 and 6, the sintering fixture according to theinvention comprises a frame 44 and compensating inserts 46 arrangedinsidethe frame 44. The compensating inserts 46 and the frame 44 bound asintering chamber for accommodating the workpiece 24. The bottom of thesintering chamber is formed by the lower die 28 of FIG. 5, and the topof the sintering chamber is formed by the upper die 26.

The geometry of the sintering fixture is shown in FIG. 6. As showntherein,the frame 44 has a length l_(A) in a first direction.Compensating inserts 46 have lengths l_(B) in the first direction. Theworkpiece 24 has a length l_(C) in the first direction. In order to beable to easilyremove the sintered workpiece 24 from the fixture afterfiring, and in order to avoid crushing or distorting the workpiece oncooling after the pressure sintering, two small gaps having lengths dare provided between the fixture and the workpiece at room temperature.The thermal expansion coefficients of the frame, the compensatinginsert, and the workpiece are α_(A), α_(B) AND α_(C), respectively.

The desired gaps between the workpiece 24 and the sintering fixture canbe provided on cooling the sintered workpiece to room temperature if thefollowing relationships hold.

    l.sub.A α.sub.A δT=2l.sub.B α.sub.B δT+l.sub.C α.sub.C δT-2d,                                (13)

and

    .sub.A =l.sub.C +2l.sub.B +2d,                             (14)

where δT is the change in temperature of the sintering fixture andworkpiece. The dimensions l_(A), l_(B), l_(C) and d are all taken atroomtemperature.

Substituting Equation 14 into Equation 13 (and recognizing the α_(A) δTis much less than 1 for sintering in the range of 1000° C.) yields;##EQU15##

The same relationship can be used to ensure a close fit between theworkpiece 24 and the sintering fixture at all temperatures in adirection perpendicular to l_(A), l_(B), and l_(C).

In order to obtain the best results, the sintering fixture should bemade of materials which resist oxidation and which resist adhering tothe workpiece. For example, the sintering fixture may be molybdenum. Thecompensating insert may be, for example, copper, nickel, or stainlesssteel. A release coating such as alumina powder may be provided onfixturesurfaces which contact the workpiece.

The operation of the sintering fixture according to the invention is asfollows. On heating, the frame 44 expands more than the workpiece 24.However, the compensating insert 46 expands more than frame 44 so as toeliminate gap d at the sintering temperature. On cooling, thecompensatinginsert shrinks more than frame 44 so the gap d reappears.

The invention will become more apparent after referring to the followingexamples.

EXAMPLE 1

A crystallizable (cordierite) glass as disclosed in U.S. Pat. No.4,301,324was ground into a powder.

The glass powder was mixed with a binder of Butvar (trademark), apolyvinylbutyral resin, dipropylglycoldibenzoate plasticizer, andmethanol/methyl isobutylketone solvent to form a slurry.

The slurry was cast into green sheets on a Mylar (trademark) substrateby doctor blading, followed by drying in air. The dry green sheets wereblanked to required dimensions, and via holes were punched in desiredconfigurations.

Metalizing paste was screen printed onto the cut green sheets to fillthe via holes. The metalizing paste was formed by mixing copper powderwith a crystallizable glass powder as described above. The copper andglass mixture was also provided with a binder of ethyl cellulose andterpineol.

A first metalizing paste contained 40 volume percent copper. A secondmetalizing paste contained 47 volume percent copper, and a thirdcontained55 volume percent copper.

Samples were produced by stacking and laminating 45 screen-printed greensheets. The laminated assemblies were heated in an H₂ /H₂ O atmosphereto burn out polymeric material and residual carbon. The burnoutwasperformed by heating at 1°-3° C. per minute to 785°±10° C., and holdingat that temperature for 3-5 hours. The ratio of H₂ to H₂ O started at10⁻⁶, and was changed continuously to 10⁻⁴ from 400° C. to the burnouttemperature. Thereafter, the atmosphere was changed to N₂ to removedissolved water, and the assemblies were cooled to room temperature at5° C. per minute.

The stacked and laminated sheets were then sintered in a nitrogen-richatmosphere of hydrogen and nitrogen as follows. The temperature of thestacked and laminated workpiece was raised by 5° C. per minute to 750°C. After holding the temperature at 750° C. for thirty minutes, theworkpiece was compressed in a sintering fixture containing a compressionstop. Some samples were compressed at 200 pounds per square inch, othersat 400 pounds per square inch, and others at 800 pounds per square inch.

Immediately after the initiation of the compression, the temperature wasraised 2° C. per minute to 870° C. The temperature of 870° C. wasmaintained with the pressure for two hours.

Finally, after two hours at 870° C., the compressive load was removed,and the temperature was decreased 5° C. per minute down toroomtemperature.

The hermeticity of each substrate was tested by applying a fluorescentdye (for example, Magnaflux) to the surface of the substrate, andallowing thefluorescent dye to penetrate the surface. After severalminutes, excess dyewas removed by rinsing and drying the surface. Thesubstrate was then let sit for a few minutes to allow any dye whichpenetrated the substrate to rise back to the surface. Each substrate wasthen examined under ultraviolet light to observe the presence of anydye.

No fluorescence was observed adjacent vias with 40 and 47 volume percentcopper, indicating good hermeticity. Fluorescence was observed at thevia/substrate interface for vias containing 55 volume percent copper,indicating the presence of structural irregularities, thereby indicatingalack of hermeticity.

Additional samples were thermally cycled between room temperature and360° C. Samples were subjected to up to 20 thermal cycles. Forviascontaining 40 volume percent copper, dye tests yielded no apparentfluorescence at the via/substrate interface, thereby indicating goodhermeticity. For vias containing 47 volume percent copper and 55 volumepercent copper, fluorescence was observed, indicating dye penetrationand therefore no hermeticity.

Electrical resistance measurements were made of the hermetic vias with40% copper using a four-point-probe technique. The average resistivitywas 35.0 microohm-centimeter.

EXAMPLE 2

A crystallizable (cordierite) glass as disclosed in U.S. Pat. No.4,340,436was ground into a powder.

The glass powder was mixed with a binder of ethyl cellulose and copperpowder to form a metalizing paste. A first metalizing paste contained 30volume percent copper. A second metalizing paste contained 40 volumepercent copper, and a third contained 55 volume percent copper.

The metalizing paste was cast into layers on a Mylar (trademark)substrate by doctor blading, followed by drying in air. Additionallayers were applied by doctor blading followed by drying until 100 milthick laminateswere obtained.

One inch square wafers were then cut from the laminates. Polymericmaterialand residual carbon was burned off in the same manner as inExample 1. The burned off laminates were pressure sintered in the samemanner as Example 1.

The resistivities of the sintered wafers were measured with a four pointprobe measurement apparatus. The results are shown in FIG. 7. In allthreecases (that is, with vias containing 30 volume percent copper, 40volume percent copper, and 55 volume percent copper), the resistivitydecreased as the compressive load increased.

It will be apparent to those skilled in the art having regard to thisdisclosure that other modifications of this invention beyond thoseembodiments specifically described here may be made without departingfromthe spirit of the invention. Accordingly, such modifications areconsideredwithin the scope of the invention as limited solely by theappended claims.

What is claimed is:
 1. A sintering fixture for pressure sintering aworkpiece having a thermal expansion coefficient α_(C), said fixturecomprising:a frame having a thermal expansion coefficient α_(A) ; and acompensating insert arranged inside the frame, said compensating insertand frame bounding a sintering chamber for accommodating the workpiece,said compensating insert having a thermal expansion coefficient α_(B) ;characterized in that α_(C) <α_(A) <α_(B).
 2. A sintering fixture asclaimed in claim 1, characterized in that:the frame has a length l_(A) afirst direction; the compensating insert has a length l_(B) in the firstdirection; the workpiece has a length l_(C) in the first direction;##EQU16## where d is a desired gap between the fixture and the workpieceat room temperature, and where l_(A), l_(B), and l_(C) are dimensions atroom temperature.
 3. A sintering fixture as claimed in claim 2,characterized in that:the frame is substantially square with four sidesof equal lengths; the fixture comprises first and second pairs ofcompensating inserts of substantially equal lengths, each compensatinginsert being arranged adjacent one side of the frame; and the workpieceis substantially square.
 4. A sintering fixture as claimed in claim 2,characterized in that l_(A) and l_(C) are diameters, and l_(B) is athickness of a ring.
 5. A sintering fixture as claimed in claim 2,characterized in that:the frame consists essentially of molybdenum; andthe compensating inserts consist essentially of one or more of copperand nickel or stainless steel.